The present invention relates to a brake-testing stand for motor vehicles comprising driven friction rollers arranged in pairs and positioned in a frame, wherein the wheel of a vehicle to be tested rolls on said friction rollers and the stator of a drive motor of the friction rollers is rotatably mounted in the frame and attached to a lever arm so that a reverse torque induced in the stator on application of a brake being tested is transmitted to the lever arm and hence to a torque-measuring or -indicating device.
Brake testing stands of the above type are known. The lever arm which is generally fixed to the stator and influences a capsule-type dynanometer (pressure cell), the corresponding measured pressure being indicated by means of a manometer. The indicating scale or the like is calibrated to indicate the measured torque value.
In general, known arrangements have two pairs of friction rollers with associated drive motors positioned adjacent to each other so that the two wheels of a motor vehicle axle can be tested simultaneously.
In the prior art arrangements, a comparatively high measuring hysteresis develops due to the fact that between the cylinder and piston of the pressure cell, friction cannot be completely eliminated and there results a difference between the dynamic and static friction. The forces for moving the piston in the cylinder from rest, due to the necessity of overcoming static friction is larger than when the piston is already in motion. The measured result with such devices is therefore not precise and is influenced by a multitude of factors which are difficult or even impossible to eliminate.
A further difficulty with the prior art brake testing stands using pressure cells results from the fact that the torque can only be measured in one direction of rotation. Even if the drive motor can be simply switched over, the dynanometer with associated devices cannot be utilized for the other rotational direction since the hydraulic arrangement is not in the position to transmit pulling forces.